Cristina Chirila

This chapter provides an overview of the versatile applications and properties of epitaxial ferroelectric materials obtained using the pulsed laser deposition technique. These materials can play a significant role in various electronic and sensing applications or energy harvesting. Materials that are ferroelectric and have a perovskite structure (ABO3 type) show spontaneous polarization that can be changed by an electric field, temperature, mechanical stress, or light. Here, we present results obtained on epitaxial ferroelectric thin films with different compositions, lead-based or lead-free, and the correlation with structural quality of the layers and with different electrostatic conditions induced either by the substrate or by the different dopants. Our studies revealed that the utilization of pulsed laser beam deposition technique is suitable for obtaining ultrathin films depositions with thicknesses measuring less than 5 nm. These results allowed us to reveal the impacts caused by polarization orientation on the band structure or the presence of self-doping phenomena. We also found that the conduction type can be modified by introducing 1% Fe and Nb on PbZrTiO₃ (PZT) epitaxial layers. In the last part of this chapter, we report on obtaining of a lead-free epitaxial thin film and its properties in the energy storage field.

Pb(Zr,Ti)O₃ (PZT) is the most common ferroelectric (FE) material widely used in solid‐state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr_0.2Ti_0.8)O₃ using soft‐X‐ray angle‐resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out‐of‐plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin‐film PZT samples, where a thickness‐induced self‐doping results in their heavy doping. For the first time, the soft‐X‐ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE‐polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO₃ and La_xSrMn_1−xO₃ substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in‐gap states below the Fermi energy. For the opposite polarization state (P−), the positive charges near the surface are identified as cation vacancies resulting from non‐ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La_0.67Ba_0.33Mn_0.95Ti_0.05O₃ (LBMTO-5) thin film was grown on SrTiO₃(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal–semiconductor transition close to this Curie temperature (T_C). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm³ K. The relative cooling power is 1400 mJ/cm³ K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to T_C. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the T_C.

Electrocaloric effect is the adiabatic temperature change in a dielectric material when an electric field is applied or removed, and it can be considered as an alternative refrigeration method. Materials with ferroelectric order exhibit large temperature variations in the vicinity of a phase transition, while antiferroelectrics and relaxors may exhibit a negative electrocaloric effect. In this study, the temperature variation in polarization was investigated for epitaxial ferroelectric thin film structures based on PbZrTiO₃ materials in simple or complex multilayered structures. We propose the intriguing possibility of a giant negative electrocaloric effect (ΔT = −3.7 K at room temperature and ΔT = −5.5 K at 370 K) in a simple epitaxial Pb(ZrTi)O₃ capacitor. Furthermore, it was shown that abnormal temperature variation in polarization is dependent on the non-FE component introduced in a multilayered structure. No significant variation in polarization with temperature was obtained for PZT/STON multilayered structures around room temperature. However, for PZT/BST or PZT/Nb₂O₅ multilayers, an abnormal temperature variation in polarization was revealed, which was similar to a simple PZT layer. The giant and negative ∆T values were attributed to internal fields and defects formed due to the large depolarization fields when the high polarization of the FE component was not fully compensated either by the electrodes or by the interface with an insulator layer. The presented results make Pb(ZrTi)O₃-based structures promising for cooling applications operating near room temperature.

The ferroelectric and pyroelectric properties of bismuth ferrite (BFO) epitaxial thin film have been investigated. The ferroelectric epitaxial thin layer has been deposited on strontium titanate (STO) (001) substrate by pulsed laser deposition, in a capacitor geometry using as top and bottom electrode a conductive oxide of strontium ruthenate (SRO). The structural characterizations performed by X-ray diffraction and atomic force microscopy demonstrate the epitaxial character of the ferroelectric thin film. The macroscopic ferroelectric characterization of BFO revealed a rectangular shape of a polarization-voltage loop with a remnant polarization of 30 μC/c m² and a coercive electric field of 633 KV/cm at room temperature. Due to low leakage current, the BFO capacitor structure could be totally pooled despite large coercive fields. A strong variation of polarization is obtained in 80–400 K range which determines a large pyroelectric coefficient of about 10⁻⁴ C/m² K deduced both by an indirect and also by a direct method.

Epitaxial La_0.7Sr_0.3MnO₃ films with different thicknesses (9–90 nm) were deposited on SrTiO₃ (0 0 1) substrates by pulsed laser deposition. The films have been investigated with respect to morpho-structural, magnetic, and magneto-transport properties, which have been proven to be thickness dependent. Magnetic contributions with different switching mechanisms were evidenced, depending on the perovskite film thickness. The Curie temperature increases with the film thickness. In addition, colossal magnetoresistance effects of up to 29% above room temperature were evidenced and discussed in respect to the magnetic behavior and film thickness.